If you haven’t seen it before, I recommend this footage showing the moment when a tribe on Papua New Guinea meets a white man for the first time in 1976. Although these people had likely seen the reflection of their own body in rivers before, look at their reactions when, for the first time, they see themselves reflected in a mirror (around min 3)! Although some of us might get just as scared when watching ourselves in the mirror in the morning, we are usually familiar with mirror-reflections of ourselves.
‘Mirror situations’ are interesting because they present a number of challenges to our brain. First, in a mirror we see ourselves from an allocentric point of view, that is the view under which we commonly see other people, and as a result the right hand looks like a left hand although it appears on the right side of space (and vice versa for the left hand). Second, when we look at ourselves in a mirror, visual and proprioceptive inputs relative to our own body parts are in conflict, that is, we see our own body at a different location (i.e. in extra-personal space) from where we feel it. Finally, visual stimuli presented near the body and seen indirectly through a mirror project the retinal image of distant objects; yet, we are able to correlate tactile sensations produced by an object (e.g. a comb through the hair) with the visual image of the object seen in a mirror.
As we find this stuff interesting we decided to explore the brain processes underlying the integration of touch with visual stimuli seen in a mirror. To do so, we exploited a paradigm that proved successful in our earlier study [1]. A group of participants received tactile stimuli coupled with visual stimuli (flashing lights), while electrical responses to the stimuli were recorded through electrodes placed over the scalp. In our previous study [1] we found that when the visual stimuli appeared next to the “touched” hand (spatially congruent) rather than next to the other hand (spatially incongruent) the electrical responses were enhanced in the early part of the brain wave (P100). Something similar has been found using electrical recordings in animals (the so-called ‘spatial rule’ of multisensory integration, e.g. [2]) but this was the first time that it was shown using EEG in humans. There are also tons of human behavioural studies that support this finding, and you can find this literature mainly under the label of ‘peripersonal space’, i.e. the space around the body. In our recent study published in Neurosci Letters [3], we found a similar ‘spatial-congruency effect’ when the visual stimuli were seen as distant mirror reflections, but this time the enhancement was on the later part of the wave (N200). Just to be sure, when the visual stimuli were actually in ‘far’ space (i.e. the out-of-reach space) at a distance that produced a retinal image comparable to that in the ‘mirror’ condition, the spatial-congruency enhancement was completely absent.
What does this all mean? That if the brain represents the peripersonal space in a privileged way, somewhat distinct from the ‘far’ space, as researchers think, then such peripersonal space representation may be appropriately “adjusted” to include mirror-reflected images of body parts (and the space around these) that appear far from the body. Although we use mirrors in everyday life, our findings suggest that such remapping of the location of visual stimuli from where we see them to where they actually are (near the body) may require some extra time.
About Chiara Sambo
Chiara is a postdoctoral research fellow, born and raised in Italy and currently living in London. She is carrying out her research at University College London and City University London, and has ongoing collaborations at King’s College London. Her main research interests lie on the cortical modulations of somatosensation by crossmodal interactions, attention, body posture etc.
When she is not busy doing research she dances and reads novels in Spanish.
References
[1] Sambo CF, & Forster B (2009). An ERP investigation on visuotactile interactions in peripersonal and extrapersonal space: evidence for the spatial rule. Journal of cognitive neuroscience, 21 (8), 1550-9 PMID: 18767919
[2] Duhamel JR, Colby CL, & Goldberg ME (1998). Ventral intraparietal area of the macaque: congruent visual and somatic response properties. Journal of neurophysiology, 79 (1), 126-36 PMID: 9425183
[3] Sambo CF, & Forster B (2011). When far is near: ERP correlates of crossmodal spatial interactions between tactile and mirror-reflected visual stimuli. Neuroscience letters, 500 (1), 10-5 PMID: 21683122
Fascinating. I am pondering as to what items I would take to show a tribe isolated from the modern World.
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